The single tif you sent works fine for me. I created 6 copies of it and used them for a cubemapped texture and all sides showed up fine. Are all the tifs the same size? Here is the script I used to test it:
Code:
import viz
viz.go()
tex = viz.add(viz.ENVIRONMENT_MAP,'Test.tif')
dome = viz.add('skydome.dlc')
dome.texture(tex)